1. Technical Field
The present invention relates generally to methods of etching, and more particularly, to methods of etching silicon nitride material.
2. Related Art
Etching of silicon nitride material has increasingly posed challenges in the semiconductor fabrication industry. One challenge of etching silicon nitride is maintaining selectivity. For example, due to integration constraints (e.g., lithography overlay issues, etches of subsequent deposited layers), a silicon dioxide mask used to etch silicon nitride is typically relatively thin (e.g., 100-200 Å), which allows limited etching. In other cases, where silicon nitride overlays a silicon dioxide, e.g., as part of a stressed nitride liner, the silicon nitride layer thickness needs to be increased to provide the required stress level to meet the device design requirements. Both these situations require a silicon nitride etch that is highly selective to the silicon dioxide. Furthermore, where silicon nitride overlays a silicide, the etch also needs to be very selective to the silicide to prevent thinning of the silicide which results in undesirable higher sheet resistance, and deposition of etched products on chamber walls, which results in chamber reliability issues. (The silicide thinning issue is magnified in a nickel silicide scheme since the silicide is typically thinner than in other schemes).
Another challenge is maintaining an isotropic etch, i.e., one in which the etch rate is the same in any direction. In particular, silicon nitride materials deposited on a substrate are not necessarily planar, which mandates an isotropic etch. Applying bias to the substrate results in an anisotropic etch, often resulting in residual nitride that forms “mini-spacers.” Thus, an isotropic process with no or minimal bias is needed to laterally etch silicon nitride.
One conventional approach for etching silicon nitride in the presence of silicon oxides uses a mixture of methyl fluoride (CH3F), tetrafluoromethane (CF4) and oxygen (O2) or a mixture of difluoromethane (CH2F2), tetrafluoromethane (CF4) and oxygen (O2), and is disclosed in U.S. Pat. No. 5,786,276 to Brooks et al. A disadvantage of this approach is that the bias voltage cannot be adjusted because it is a chemical downstream etching process.
In view of the foregoing, there is a need in the art for a silicon nitride etching method that does not suffer from the problems of the related art.